Typically, polyimide is prepared through the thermal or chemical imidization of polyamic acid, a precursor of polyimide, which can be obtained by reacting dianhydride with diamine in an organic solvent.
Because of its excellent in thermal resistance, chemical resistance, electrical insulation, and mechanical properties, polyimide resins find numerous applications in the electric and electronic appliance, adhesive, composite material, fiber, and film industries.
By virtue of its linear backbone structure which allows chains to be packed at a high density as well as the rigidity of the imide ring itself, polyimide can show superior thermal resistance. Particularly, the polyimide which is specialized to be used in areas where high temperature stability is required, such as in the production of films, has a linear backbone structure such that the packing density of polymer chains is high, largely determining the thermal resistance of the polyimide. Commercially available polyimide films, exemplified by Kapton and Upilex, typically exhibit such structures. Kapton is known to be prepared from pyromellitic dianhydride (PMDA) and oxydianiline (ODA) monomers while Upilex can be prepared from 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) and para-phenylenediamine (PPD) monomers.
No mater how improved it is, the thermal resistance of such linear structures falls within the scope of the conventional polyimide films. An increase in molecular weight of a polyimide film with the aim of improving its thermal properties results in deteriorating its mechanical properties such as flexibility. Various attempts have been made to improve the thermal resistance of polyimide.
For example, Japanese Pat. No. 63-254131 recruits PPD into the polyimide structure prepared from PMDA and ODA, such as Kapton. However, the resulting film suffers from a disadvantage of being poorer in mechanical strength as the content of PPD is higher. In addition, there is found to exist a limit of increasing only the thermal resistance without deteriorating the mechanical properties.
Another technique to overcome the problems that linear structures of polyimide have can be referred to U.S. Pat. No. 5,231,162 in which tri- or tetra-amine is introduced into aromatic diamine with the aim of converting the linear structure into a three-dimensional molecular structure through gelation, whereby both the thermal properties and the mechanical properties can be improved. Resulting from the use of aromatic tetracarboxylic dianhydride and aromatic diamine only, a deficiency in flexibility is found in the film. In addition, the gelation causes the polyamic acid and the polyimide to decrease in solubility, thus making the processability of the resins poor.
With respect to the polyamic acid or polyimide which is used as an adhesive material, its adhesiveness at high temperatures is dependent on the flowability at such temperatures. Introduction of ether into the main chain of the polymer brings about a decrease in its glass transition temperature, thus enabling an improvement in the high-temperature adhesiveness, but a decrease in its thermal resistance, as well, thus making the reliability poor for a process which is carried out with the adhesive.
U.S. Pat. Nos. 4,847,349 and 5,406,124, disclose thermoplastic adhesives based on the diamine containing two or more ether groups which are introduced among three or four phenyl rings. The adhesives show an improvement in high-temperature flowability, but suffer from a disadvantage of being lower in thermal resistance.